Bluetooth™ is a wireless technology designed to allow instant, short-range digital connections to be made between different electronic devices, replacing the cables that connect current devices. The Bluetooth radio is built into a microchip and operates in the unlicensed 2.4 GHz band. Frequency-hop transceivers are used to combat interference and fading. Information is exchanged in packets, with each packet transmitted on a different hop frequency. The data bits are encoded using Gaussian Frequency Shift Keying (GFSK), a shaped, binary frequency modulation scheme aimed at minimizing transceiver complexity. The specified symbol rate is 1 million symbols per second (1 Ms/s). Technical aspects of Bluetooth are described in detail in the Bluetooth specification, which is available at www.bluetooth.com and is incorporated herein by reference.
A receiver for use in a Bluetooth environment amplifies signals that it receives from the transmitter and downconverts them to baseband for demodulation. The amplification is typically controlled by an automatic gain control (AGC) circuit, as is known in the art. In addition, the receiver preferably generates a receiver signal strength indicator (RSSI) signal, which is returned to the transmitter for use in controlling the transmitted power. To demodulate the signals, the receiver must be synchronized with the timing and frequency offset of the transmitted signal. For this purpose, Bluetooth packets have an access code header that includes a 64-bit synchronization word (sync word). The receiver must detect the sync word in order to generate the required timing and frequency adjustments. Once the receiver is synchronized, it demodulates the digitized signals to recover the binary symbol stream of ones and zeroes.
Various methods are known in the art for demodulation of frequency shift-keyed (FSK) signals. The simplest method (which has been implemented in early Bluetooth receivers) is analog discrimination of the frequency changes in the signal. Coherent, digital demodulation methods, however, provide better performance in conditions of low signal/noise ratio (SNR) and high intersymbol interference (ISI). An exemplary method of this type is described by Morelli et al., in “Joint Phase and Timing Recovery with CPM Signals,” published in IEEE Transactions on Communications 45:7 (1997), pages 867-876, which is incorporated herein by reference. The authors propose a general method for processing continuous-phase modulation (CPM) signals, noting that the method is particularly advantageous in continuous-phase frequency-shift keying (a class of FSK that includes GFSK). According to the method of Morelli et al., digitized signals are processed using a maximum likelihood method to determine the correct carrier phase and timing. The phase and timing are then used in coherent demodulation of the signals.